[libnotificationmanager] introduce the WatchedNotificationsModel

Summary: This allows one to subscribe to notifications from notification server.

Test Plan:
tested using very simple QML

```
import QtQuick 2.0
import org.kde.notificationmanager 1.1 as Notifications

...
Notifications.NotificationWatchedModel {
    id: model
}

```

Reviewers: #plasma, broulik, davidedmundson

Reviewed By: #plasma, broulik

Subscribers: nicolasfella, plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D28509